The present invention relates to improvements in apparatus and methods for collection and removal of solids from slurry bodies undergoing agitation and, more particularly, concerns an elutriation apparatus and method characterized by substantially improved efficiency of operation. In this regard, an important aspect of the present invention is specifically directed to an improved elutriation apparatus and method especially suited for evaporator and crystallizer systems wherein the transfer of agitational forces from a main vessel to an elutriation leg directly connected thereto is effectively minimized enabling a body of slurry in the vessel to be subjected to high internal circulation and agitation without disrupting or restricting the formation and maintenance of a fluidized bed in the elutriation leg.
Elutriation legs or columns are commonly of elongated cylindrical construction having an open-ended top in direct communication with a body of slurry contained in a main vessel or evaporating chamber and a bottom end which is normally closed with a dished head or cone. Feed or elutriating liquor is supplied to the bottom of the leg often through a distribution system to obtain an even upward flow, and travels upwardly through the leg. Simultaneously, slurry is gravitationally fed to the top of the elutriation leg and travels downwardly therethrough in countercurrent flow to the elutriating liquor flow for removal from the leg at a location near the bottom end thereof. The upward flow of elutriating liquor is used to fluidize the downward flow of solids.
While elutriation legs are employed in a number of different applications, they are particularly useful in evaporator and crystallizer systems which include a main vessel containing a slurry body undergoing evaporation or cooling wherein salt crystals are formed and then collected and removed in the elutriation leg which is directly connected to the main vessel in pendant relationship therewith. In such evaporator and crystallizer systems, the formation of an efficient fluidized bed in the elutriation leg results in advantageous thickening of the discharged slurry as well as washing and classifying of the salt crystals.
The formation of a thickened or heavy salt discharge is important since it enables removal of maximum amounts of salt crystals with minimum amounts of liquor. Correspondingly, efficient washing of the salt is important since it results in the displacement of body liquor from the salt crystals with elutriating liquor. Normally, the elutriating liquor is weaker in dissolved impurities than the body liquor and, accordingly, displacement of such body liquor from the crystals with the elutriating liquor produces a more desirable crystal product. The classifying ability of an efficiently operating elutriation leg not only offers the advantage of enabling the discharge of larger crystals from the leg but also can be used to classify different types of salt which are present in a given system. For example, in an efficiently operated elutriation leg connected to a salt evaporator, calcium sulfate crystals can be effectively separated from sodium chloride crystals with the result that the finer calcium sulfate crystals are returned back into the slurry body in the main vessel with the elutriating liquor flow.
Agitation in the evaporator or crystallizer main vessel such as, for example, is caused by forced circulation with a pump or agitator, as well as natural circulation, is essential to providing desirable operating conditions in the main vessel body. Transfer of such agitational forces to the elutriation leg, however, produces a disruption and/or restriction in the formation and maintenance of a fluidized bed and, as such, drastically reduces the effectiveness of the elutriation leg.
The present invention provides a method and apparatus by which the transfer of agitational forces in a slurry body contained in a main vessel to an elutriation leg directly connected therewith is effectively minimized so as to provide substantially improved efficiency of such leg and enable the formation and maintenance of a fluidized bed therein.
In accordance with an important aspect of the present invention, an elutriation leg is provided with a novel slurry inlet and liquor outlet device in the upper portion thereof which effectively minimizes the transfer of agitational forces from the main vessel to the elutriation leg. This device includes a slurry flow directing member (preferably of vertically disposed, frustoconical tubular or cylindrical configuration) which receives slurry flow from a main vessel directly connected thereto and discharges such flow in a downward flow path of reduced cross-sectional area with respect to the cross-sectional area of the elutriation leg onto a target member mounted below the slurry flow directing means. The target member is preferably a dish-shaped horizontally disposed component which functions to direct the downward flow of slurry in a radially outward direction to substantially minimize the concentrated downward force of the slurry that would otherwise tend to disrupt the efficient operation of the elutriation leg. Liquor flow directing means located above the target member is provided through which liquor which is relatively free of solids is directed for passage into the main vessel. If desired, a swirl breaker can be provided for dissipating rotational forces imparted to the slurry in the main vessel. In a preferred embodiment, a suitable baffle member, operatively associated with the liquor flow directing means, functions to prevent or at least minimize reverse slurry flow therethrough.
It is, therefore, an object of the present invention to provide an improved apparatus and method for the collection and removal of solids from slurry bodies undergoing agitation.
Another object of the present invention is to provide an improved elutriation apparatus and method characterized by improved efficiency of operation.
Another object of the present invention is to provide an improved elutriation apparatus and method wherein the transfer of agitational forces in a slurry body contained in a main vessel to an elutriation leg directly connected thereto is effectively minimized, enabling a body of slurry in such vessel to be subjected to high internal circulation and agitation without disrupting or restricting the formation and maintenance of a fluidized bed in the elutriation leg.
Another object of the present invention is to provide an improved elutriation apparatus and method wherein the downwardly directed slurry flow forces adjacent the upper end of an elutriation leg are deflected radially outwardly, thereby substantially dissipating those forces which would tend to disrupt the formation and maintenance of a fluidized bed therein.
Another object of the present invention is to provide an improved elutriation apparatus and method wherein passage means is provided for discharge of elutriating liquor from the elutriation leg into a slurry body contained in a main vessel, which passage means is also equipped to at least minimize reverse flow of such slurry from the vessel to the elutriation leg therethrough.